Fluidized bed reactor and distribution system

ABSTRACT

This invention provides a fluidized bed reactor with improved method and apparatus for inhibiting and clearing clogging of the fluid distribution network in such reactor. In particular, at least one channel or reservoir of fluid is provided over at least a selected portion of the distribution network, with fluid from such column/reserve being released to maintain flow and pressure in the distribution network as the media bed is defluidized. For the preferred embodiment, the fluid column is (a) a feed pipe connecting a fluid inlet which is above the reactor fluid level to the distribution network; and (b) a plurality of risers which, for the preferred embodiment, extend from the end of each lateral tube of the distribution network. A vacuum breaker is provided for each fluid column to permit fluid to flow from the column into the network when pressure is removed. A valve controlled opening may be provided at the top of each riser to permit purging of the distribution network through the openings when the valve is opened and to provide access to the distribution network for clearing clogs without requiring removal of the media bed.

FIELD OF THE INVENTION

This invention relates to fluidized bed reactors and more particularlyto an improved fluid distribution system for such reactors.

BACKGROUND OF THE INVENTION

Fluidized beds of sand, activated carbon, or other media have been usedfor a wide variety of aerobic and anaerobic waste treatment processes.Because of their extremely high surface areas for biological growth andhigh hydraulic loading rates, fluidized beds have demonstratedsignificant cost and performance advantages for biological treatment ofindustrial, process, and municipal waste water. Nevertheless, asignificant number of these systems which were built over the lasttwenty years have become inoperable or have been subject to excessivemaintenance. These operational problems can be traced to the chronicclogging of the inlet distribution system which distributes the influentat the bottom of the filter bed.

The inlet distribution system of these biological fluidized beds canbecome clogged for two principal reasons. First, cessation of normalflow through the filter causes intermittent fluid backflow. Thiscondition draws the media bed into the inlet distribution system andcauses clogging. The problem occurs because the media has a greaterdensity than the fluid and therefore tends to displace the fluid in, forexample, the lateral elements of the distribution plumbing. Conventionalcheck valves which attempt to prevent this require some degree ofbackflow in order to activate them. They are thus ineffective inpreventing clogging of the inlet system. Moreover, conventional checkvalves are subject to sticking when even small amounts of media becomecaught in them. The second, principal cause of clogging occurs whenforeign materials become lodged in the laterals or openings in thedistribution system and impedes normal operation.

A third factor may play a role in clogging the laterals. The flow in thelaterals is turbulent. Pressure fluctuations associated with such flowconditions appear to draw media into the laterals even under normaloperating conditions.

Since it is critical to maintain an even flow distribution through thesystem so that the bed is uniformly fluidized, even a few nozzlesbecoming inoperative permits the media above them to settle into a heavydense unfluidized mass. The remaining nozzles receive more flow and, asa result, the media above them becomes even more fluidized and tends tobe deposited over the dead zone, increasing its size and making theproblem worse. This phenomenon of excessive fluidization and subsequentmedia deposition typically worsens until a major part of the bed is nolonger fluidized.

Since the distribution plumbing is typically under one to severalhundreds of tons of media, clearing media from the reactor to unclogeven a few nozzles is a maintenance nightmare, being difficult, messy,time-consuming and expensive.

SUMMARY OF THE INVENTION

In accordance with the above, this invention provides a fluidized bedreactor for filtering fluids and a method for inhibiting the clogging ofa fluid distribution network of such a reactor. More particularly, thereactor has a tank containing a media bed. The reactor also has fluidinlet to the tank, a fluid outlet from the tank and a distributionnetwork connected to receive fluid from the inlet and having outletports or nozzles for the substantially even distribution of fluidthrough the media bed. The outlet maintains a selected fluid level inthe tank, which level is above the level for the media. A means is alsoprovided for temporarily maintaining a fluid flow through at least aportion of the distribution network after a termination in fluid flow tothe distribution network from the fluid inlet.

The technique for temporarily maintaining fluid flow generally involvesproviding at least one fluid reserve or column, with each such reservecolumn being positioned over a selected portion of the distributionnetwork. The reserve/column fluid is permitted to flow into and throughat least the corresponding selected portion of the network when there isa termination in fluid flow from the inlet.

More specifically, two separate techniques are disclosed for thetemporary maintenance of fluid flow, which techniques can be usedindividually, but are used together for the preferred embodiment. In onetechnique, at least one riser is provided with each riser extendingsubstantially vertically from a selected portion of the distributionnetwork to a height above the fluid level in the tank. Each riser isconnected to receive flow fluid from the network with fluid normallyrising in the riser to a level above the fluid level in the tank whenthere is fluid flow in the network. For the preferred embodiment, thedistribution network includes a plurality of spaced lateral tubes whichextend from opposite sides of a conduit connected to receive fluid fromthe fluid inlet. Each lateral tube has a plurality of spaced ports ornozzles for delivering fluid to the media and has at least one riserextending therefrom, the riser preferably extending from a point nearthe end of the lateral tube.

A valve controlled opening is preferably provided near the top of eachriser. The distribution network in general, and the lateral tubeconnected to the riser in particular, may be purged of media materialwhich has migrated into the tube by opening the valve, permitting fluidin the riser and in the lateral tube to gush from the resulting opening,carrying with it any media material from the lateral tube. The openingalso permits access to the lateral tube and its nozzles withoutrequiring removal of media from the tank.

A small orifice may also be provided near the top of each riser. Thisorifice serves a number of functions, including serving as an escapehole for trapped air so that fluid may rise in the risers, and servingas a vacuum break to permit fluid from the riser to flow back into alateral tube. Finally, a small quantity of fluid may spray from theorifice, providing a visual indication that the riser is full.

The second way in which the flow maintaining function may be performedis for the fluid inlet to enter the tank at a level above the fluidlevel in the tank and to be connected to the distribution networkthrough a substantially vertical pipe. A vacuum breaker, for example alarge float valve, is provided near the fluid inlet so that, when thereis an interruption in fluid flow to the inlet, the fluid in the pipe mayflow into the distribution network to temporarily maintain fluid flow tothe network.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view of a fluidized bed reactorin accordance with a preferred embodiment of the invention.

FIG. 2 is a cross-sectional view of the reactor taken generally alongthe line 2--2 in FIG. 1.

FIG. 3 is a plan cross-sectional view of the reactor taken generallyalong the line 3--3 in FIG. 1.

DETAILED DESCRIPTION

The fluidized bed apparatus or reactor of the invention is applicable tothe removal of contaminants and other compounds or matter from a fluidsuch as water. Liquids from wastewater treatment plants, activated sludeprocesses, aquaculture tanks, and fish farming ponds, are examples ofwaters that might be treated with the apparatus.

The fluid to be treated can contain a variety of compounds, includingammonia, amines, nitrites and nitrates. Fluids high in ammonia may besubjected to oxidation and the action of aerobic bacteria that live onthe media. The reactor can also be used with denitrifying bacteria toconvert oxidized nitrogen compounds, i.e., nitrates and/or nitrites) tonitrogen gas.

The "fluidized bed" refers to the flow of fluid (and the materials to beremoved from it) through a bed of fine particles. The velocity of thewater or other fluid is chosen to slightly expand the bed of particles(by, for example, 30%) and to impart to them a gentle movement. When inthis state, the media may be referred to as being "fluidized". Themicroorganisms to cause the desired change in the water may come from anatural contamination or they may be deliberately added.

Referring to the Figures, a fluidized bed reactor 10 is shown whichincludes a reactor vessel of tank 12 having sidewalls 14 and a base 16connecting the sidewalls. A fluid inlet pipe 15 is provided fordelivering fluid to the reactor. Inlet pipe 15 preferably extends overwall 14 and into the reactor where it is jointed to a fluid deliverypipe 11 in fluid communication with a distribution network 13. A vacuumbreaker/air relief valve 17 is located at the top of inlet pipe 15, at,for example, the junction of this pipe with pipe 11. Valve 17 may, forexample, be a float valve which is closed when there is fluid pressureapplied to inlet 15 and which opens when such fluid pressure is removed.

Distribution network 13 is positioned near the bottom of tank 12 andincludes a horizontal main conduit 42 extending along a diameter of thetank (FIG. 3) which is connected to receive fluid from delivery pipe 11and a plurality of horizontal or lateral tubes 44 extending at rightangles from both sides of conduit 42. Each lateral tube 44 (sometimeshereinafter referred to as a lateral) has a plurality of downward-facingports or nozzles The nozzles provide a sufficient pressure drop foruniform flow distribution through media bed 18. Since nozzles 46 aredirected downward, the fluid flows down out of the nozzles. However,since the fluid cannot flow through the solid floor 16 of the tank, itreverses direction and flows up through the media bed. The desiredmicroorganisms are normally innoculated into the system and subsequentlygrow on the media. As the fluid flows upward through media bed 18, itcomes in intimate contact with the media and the desired filteringprocess is effected. As previously indicated, the velocity of the fluidis such that it expands the media bed, the expanded media bed being, forexample, at the level 20. When fluid pressure is removed from thedistribution network, the media bed settles back to an unexpandedcondition over a period of time which is generally a few seconds.

After passing through the media bed, the fluid continues to flow upwardwith the filtered fluid exiting the tank through fluid outlet 24. Outlet24 controls the fluid level 22 in the reactor, although, depending onthe actual flow conditions, the steady state reactor fluid levelmaintained within the reactor may be at a height above that of fluidoutlet 24.

Fluid is exhausted from the reactor 10 by way of outlet 24 after passagethrough the media bed 18. Selected portions of the exhausted fluid maybe recycled through conduit 26 as required for complete filtering or topromote further growth of the biocatalyst on the media bed. Anadditional conduit 28 can be connected to the fluid inlet 15 to provideadditional oxygen sources and/or carbon, nitrogen or other nutrients forthe biocatalyst in sufficient amounts to satisfy growth and metabolismrequirements of the biocatalyst. The metering of sufficient amounts ofthese additional sources may be conducted automatically by providingconventional nutrient and/or gas analyzers which may be adapted toperiodically sample the influent fluid and determine its nutrient and/orgas content.

A plurality of tubes 30 (hereinafter referred to as "riser tubes" or"risers") are in fluid communication with the distribution network 13.These riser tubes can be located outside tank 12, although theypreferably are located within the tank. Each riser tube has one endconnected to the distribution system 13 and another end 34 that islocated above both the fluidized media bed level 20 and the reactorfluid level 22. Upper end 34 of each riser tube is normally closed by avalve 52 and a small opening or orifice 36 is provided in the riser somedistance below the closed end and above reactor fluid level 22. For apreferred embodiment, opening 36 is approximately 1/4. A small quantityof fluid 38 may spray from orifice 36, indicating that the riser isfull. Valve 52 may be a knife-gate valve. Upper end 34 of each riserextends over tank 12 so that when valve 52 is in an open position, fluidflows through the corresponding lateral tube 44 and riser 30 and outvalve 52 into the tank.

Tank 12 can be made of a variety of materials, the material utilizedpreferably being inert (i.e., not chemically or physically altered) tothe to the biocatalyst and inert to the particular fluid flowing throughthe reactor. Exemplary materials for constructing the tank and othercomponents of the reactor include noncorrosive metals such as stainlesssteel, polymers such as polyvinylidene chloride, polyethylene,polytetrafluorethylene, polypropylene, fiberglass-reinforced plastic,and other materials such as concrete. Particularly preferred materialsfor the tank are fiberglass-reinforced plastic.

Suitable materials for the media bed include natural or artificialsubstances such as coal, volcanic cinders, glass or plastic beads, sand,activated carbon particles, and alumina. The size of the individualparticles of the media bed are a function of both their specific gravityand surface area. For the most part, it is preferred to employ mediaparticles of between about 0.1 to 10 mm in diameter. Most preferably,the particles are of a uniform size. While the above media bed materialsare illustrative of the preferred materials which are useful, othermaterials inert to the biocatalyst, either natural or synthetic, can beemployed.

Fluid flow through the media bed is preferably kept within a narrowrange of flow rates; the range being primarily a function of the type,size and specific gravity of the media bed, type of fluid and reactorvessel size. Exemplary flow rates in the preferred embodiments should besufficient to produce an approximately 30% expansion in volume of themedia above the media bed volume at no flow.

A "biocatalyst", as defined herein, is a prokaryotic and/or eukaryoticorganism, preferably a microorganism, that can convert organic andinorganic materials in the fluid to other products. For example, and notby way of limitation, biocatalysts can include nitrifying bacteria(e.g., Nitrosomonas, Nitrobacter) that can convert ammonia or amines tonitrates or nitrites. Protozoans and/or fungi are other examples ofbiocatalysts that can be used to perform a particular transformation.The term "biocatalyst" is also meant to include recombinantly derivedmicroorganisms, fragments of these microorganisms, and enzymes isolatedfrom these microorganisms. It will be understood that the exact kind andquantity of biocatalyst is not intended to limit the scope of thepresent invention. The biocatalyst can be introduced by the operatorinto the media bed as a substantially pure or mixed culture ofmicro-organisms. The biocatalyst can also be introduced into the mediabed, and can colonize the media bed, by way of the fluid itself.

In operation, all of the valves 52 are normally closed and fluid underpressure is applied to fluid inlet 15. The fluid in inlet pipe 15 causesvalve 17 to be closed. This fluid flows through pipe 11 and conduit 42to the laterals 44. The fluid pressure is sufficient so that the fluidentering laterals 44 is passed through nozzles 46 into media bed 18 tofluidize the bed. The fluid pressure in laterals 44 also causes fluid torise in risers 30 to the level of valves 34. While some fluid spraysfrom orifices 36, the orifices are small enough, being for example 1/4inch in diameter for a 4 inch diameter riser, that the orifices do nothave a significant adverse affect either on the height or pressure offluid in the risers.

The steady-state condition described above continues until there is atermination of fluid flow at inlet 15. When this occurs, fluid pressurefrom nozzles 46 terminates and the media 18 settles back from itsfluidized to its unfluidized state over a period of time which normallytakes several seconds. It has been found that since there is an absenceof fluid pressure at nozzles 46 during this period while the media isstill active, this is the period when media is most likely to migratefrom the nozzles into laterals 44, causing a potential clogging of thenozzles and in some instances a clogging of the laterals themselves. Theproblems resulting from such clogging have been previously described.

Therefore, in accordance with the teachings of this invention, amechanism has been provided for providing a temporary continuance offluid flow and pressure at the nozzles during the defluidization periodof the media to minimize the likelihood of media entering the nozzlesand laterals during this period. In particular, this objective isachieved both by inlet feed tube 11 and by risers 30. More specifically,when fluid flow at inlet 15 terminates, there is a volume of fluid inpipe 11 which, depending on the length of the pipe and its diameter, maybe 10 to 20 gallons. The absence of fluid in inlet pipe 15 permitsvacuum break valve 17 to open, allowing air to enter pipe 11 andpermitting the fluid in this pipe to flow into conduit 42 and throughlaterals 44 to the nozzles. The opening at vacuum release 17 and theflow resistance in distribution network 13 as a result, for example, ofnozzle size will determine the rate at which fluid flows from pipe 11.These factors may be controlled so that the time period required forpipe 11 to completely empty is roughly equal to the settling time ofmedia 18 so that fluid pressure is maintained at the nozzles duringsubstantially the entire fluid settling time, minimizing the migrationof media into the nozzles and laterals.

Similarly, when flow from inlet 15 terminates, there may also be severalgallons, for example, 2 gallons, of water or other fluid in each of therisers 30. Orifices 36 serve as vacuum breakers to permit this fluid toflow back into the corresponding laterals to maintain pressure thereinas the media bed settles. Again, the time it takes for fluid to flowfrom the riser will depend on the height of the riser above the fluidlevel 22 in tank 12, on the size of orifice 36 and on the back pressurefrom the distribution network. These factors can be selected to maintaina desired fluid pressure in network 13 until the media bed has becomesubstantially defluidized.

A technique has thus been provided for maintaining a sufficient fluidflow in the distribution network of a fluidized bed reactor duringdefluidization of the reactor to minimize migration of media into thenozzles and laterals of the distribution network during this period. Theapparatus of this invention also may be utilized to purge media whichhas migrated into the distribution network and to provide access to thenetwork without requiting the nightmarish process of removing the mediabed. In particular, the valves 54 may be periodically opened, forexample once a week or once every two weeks, while fluid pressure isbeing applied to the system from inlet 15. Since there is some pressurein the risers against valve 52 at this time, opening of the valvepermits fluid in the risers and at least in the lateral to which a riseris attached to gush from the opening 34, which may for a preferredembodiment be a relatively large opening, for example four inches,carrying any media which has migrated into the lateral with it. Thefluid and media exiting end 34 of each riser are returned to tank 12.

Normally, this purge operation will remove a sufficient quantity ofmedia which may have accumulated in a lateral 44 behind any nozzle 46 sothat when valve 52 is closed, causing the lateral and nozzles to berepressurized, any remaining media at the nozzles will be forced out bythe fluid pressure, completely freeing the nozzles. However, there aresituations where a blockage in a lateral or the clogging of a nozzle maybe such that the purge and repressurization operation described above isnot adequate to fully unclog the system. In those situations, the valve52 for the riser connected to the lateral which is clogged or thelateral containing clogged nozzles may be fully opened and either aflexible reaming device or brush may be passed through the riser intothe lateral to loosen the caked media so that it may be removed during asubsequent purge. Alternatively, a hose may be snaked into the riser toan appropriate position to direct high pressure water or other fluidinto the lateral to clear the clog or some other suitable implement maybe manipulated through the open end 34 of the riser and the riser to thelateral to clear the clogs. The important thing is that an easilyaccessible access port is provided to the laterals and nozzles to permitclogs to be cleared without requiring removal of the media bed.

While the invention has been described above with respect to a preferredembodiment, it is apparent that with a suitable design of the risers 30,inlet 15 could be brought in at the bottom of the tank and pipe 11eliminated. Similarly, in a suitable system, pipe 11 might be sufficientto maintain fluid flow and pressure during media settling without risers30. However, the removal of risers 30 would eliminate the advantagesprovided by these risers in terms of purging media from the laterals andpermitting access to the laterals and nozzles without removal of themedia bed for the removal of clogs. Further, while a particular reactorconfiguration has been shown in the Figures, it is apparent that theteachings of this invention could be utilized with other reactorconfigurations. Further, while specific dimensions have been provided insome instances, these dimensions are by way of illustration only and thedimensions would vary with the system utilized. Finally, while twotechniques have been disclosed for providing a fluid channel or reservewhich can flow back into the distribution network when inlet fluidpressure is removed, other configurations for providing such column orreserve of fluid might also be employed. Thus, while the invention hasbeen particularly shown and described above with reference to apreferred embodiment, the foregoing and other changes in form and detailmay be made therein by one skilled in the art without departing from thespirit and scope of the invention.

What is claimed is:
 1. A fluidized bed reactor for filtering fluidscomprising:a reactor tank defining a volume; a media bed disposed withinsaid tank to a selected level; a fluid inlet to said tank; adistribution network .Iadd.within the media .Iaddend.connected toreceive fluid from said inlet and having outlet ports for distributionof the fluid through the media bed; a fluid outlet from said tank, saidoutlet maintaining a selected fluid level in the tank, which level isabove that of the media; and flow means for temporarily maintaining agravity fed fluid flow through at least a portion of said distributionnetwork after a termination in fluid flow from the fluid inlet.
 2. Areactor as claimed in claim 1 wherein said flow means includes at leastone riser, each riser extending substantially vertically from a selectedportion of said network to a height above said selected fluid level andbeing connected to receive fluid from said network, fluid normallyrising in said risers to a level above said selected fluid level whenthere is fluid flow to said network.
 3. A reactor as claimed in claim 2wherein said distribution network includes a plurality of spaced lateraltubes, each having a plurality of spaced ports, and wherein there is atleast one riser extending from each of said lateral tubes.
 4. A reactoras claimed in claim 3 wherein said network includes a main conduitconnected to receive fluid from said fluid inlet, and a plurality oflateral tubes extending from opposite sides of said conduit for aselected length, and wherein the riser extending from each lateral tubeextends from a point on the tube near the end of the selected length. 5.A reactor as claimed in claim 3 including an opening near the top end ofeach riser, and valve means for normally closing said openings, saidvalve means, when open, permitting purging of at least a portion of thenetwork through the corresponding opening.
 6. A reactor as claimed inclaim 3 including a small orifice near the top end of each riser.
 7. Areactor as claimed in claim 1 wherein said fluid inlet enters said tankat a level above said selected fluid level, and wherein said flow meansincludes a pipe connecting said fluid inlet to said distributionnetwork, and vacuum breaker means operative after said termination offluid flow for permitting fluid in said pipe to flow into saiddistribution network.
 8. A reactor as claimed in claim .[.6.]. .Iadd.7.Iaddend.wherein said flow means further includes at least one riser,each riser extending substantially vertically from a selected portion ofsaid network to a height above said selected fluid level and beingconnected to receive fluid from said network, fluid normally rising insaid risers to a level above said selected fluid level when there isfluid flow to said network.
 9. A reactor as claimed in claim 1 whereinsaid flow means includes means for providing at least one column offluid connected to and extending above said distribution network, theheight of each of said fluid columns being greater than said selectedfluid level in said reactor; andmeans for permitting each column offluid to flow into the distribution network after said termination offluid flow.
 10. A method for inhibiting clogging of the fluiddistribution network .Iadd.within the media bed .Iaddend.of a fluidizedbed reactor comprising the steps of:providing at least one column offluid connected to and extending above said distribution network, theheight of each of said fluid columns being greater than the fluid levelin said reactor; and permitting each column of fluid to flow into thedistribution network in the event there is a termination of normal fluidflow to said network.
 11. A method as claimed in claim 10 including thestep of providing a vacuum breaker for each column at least when saidtermination of normal fluid flow occurs.
 12. A method as claimed inclaim 11 wherein said method includes the step of purging the network ofmedia which has entered the network from the fluidized bed.
 13. A methodas claimed in claim 12 wherein said providing step includes providing atleast one riser extending from the network, which riser is normallysealed at its upper end, and said purging step includes the step ofopening the sealed end of the riser, permitting a quantity of fluid fromthe riser and the network to gush out through the resulting opening. 14.In a fluidized bed reactor of a type having a tank containing a mediabed to a selected level through which fluid is to be passed forfiltering, and a fluid outlet which at least in part maintains aselected fluid level in the tank, which level is above the selectedmedia level, the improvement comprising a distribution systemcomprising:a fluid inlet to said tank; a distribution network.Iadd.within said media bed .Iaddend.connected to receive fluid fromsaid inlet and having outlet ports for the distribution of the fluidthrough the media bed; means for maintaining at least one fluid reserve,each of said reserves being positioned over a selected portion of saidnetwork at a level above said selected fluid level; and means forpermitting said reserves to flow into at least the correspondingselected portion of the network after a termination in fluid flow fromsaid fluid inlet. .Iadd.
 15. A fluidized bed reactor for filteringfluids comprising:a reactor tank defining a volume; a media bed disposedwithin said tank to a selected level; a fluid inlet to said tank; adistribution network within said media bed connected to receive fluidfrom said inlet and having outlet ports for distribution of the fluidthrough the media bed; a fluid outlet from said tank, said outletmaintaining a selected fluid level in the tank, which level is abovethat of the media; and risers passing through and extending to aselected height above said media bed from a plurality of points on saiddistribution network, said risers controlling media clogging of saiddistribution network. .Iaddend..Iadd.16. A reactor as claimed in claim15 including a vacuum breaker for each riser for permitting air toescape when there is pressure in the distribution network so that fluidcan rise in said risers and for permitting air to enter the riser sothat fluid can flow from the riser into the distribution network whenpressure drops in the distribution network. .Iaddend..Iadd.17. A reactoras claimed in claim 15 including an opening near the end of each riser,and a element for normally closing said openings, said element, when notclosing the opening, permitting purging of at least a portion of thenetwork through the corresponding opening to flush media from thenetwork. .Iaddend..Iadd.18. A reactor as claimed in claim 15 including anormally closed access opening near the top of each riser, each of saidopenings, when open, permitting access to at least a portion of thedistribution network to clear clogs therefrom. .Iaddend..Iadd.19. Areactor as claimed in claim 18 including at least one of a high pressurefluid source, a reamer and a brush which is passed through said openingand the corresponding riser to clear selected clogs. .Iaddend..Iadd.20.A fluidized bed reactor for filtering fluids comprising:a reactor tankdefining a volume; a media bed disposed within said tank to a selectedlevel; a fluid inlet to said tank; a distribution network within saidmedia bed connected to receive fluid from said inlet and having outletports for distribution of the fluid through the media bed; a fluidoutlet from said tank, said outlet maintaining a selected fluid level inthe tank, which level is above that of the media; and at least one riserconnected to receive flow fluid from said distribution network, saidriser extending substantially vertically from a selected portion of saiddistribution network through said media bed to a height which is atleast above said selected level of the media bed. .Iaddend..Iadd.21. Afluidized bed reactor according to claim 20 comprising a plurality ofsaid risers. .Iaddend..Iadd.22. A fluidized bed reactor according toclaim 20, wherein said at least one riser extends above said selectedfluid level, and further comprising at least one opening in each said atleast one riser above the selected fluid level in said tank near the topof the riser. .Iaddend..Iadd.23. A fluidized bed reactor according toclaim 22, wherein said at least one riser comprises a plurality ofopenings above the selected fluid level and at least one of saidopenings is a valve controlled opening near the top of said riser abovethe selected fluid level in said tank. .Iaddend.